Sensors for the detection of explosives are important for various disciplines including humanitarian de-mining, remediation of explosives waste sites, homeland security, and forensic applications. Different sensors for analyzing explosives were reported in the past decade. These included optical sensors where the fluorescence of functional polymers was quenched by the nitroaromatic compounds [1,2] luminescent polymer nanoparticles, such as polysilole, that were quenched by trinitrotoluene (TNT) [3] or fluorescent silicon nanoparticles that were quenched by nitroaromatic vapors [4].
The detection of more hazardous explosives such as hexahydro-1,3,5-trinitro-1,3,5-trizine (RDX) or pentaerythritol tetranitrate (PETN) is significantly less developed, and of need of further efforts, particularly the improvement of the sensitivities associated with the analysis of these substrates [5,6]. Different optical, electrochemical, or microgravimetric sensors or biosensors for TNT were reported. Fluorescent organic polymers which are quenched by nitroaromatic explosives [7,8], luminescent polysilole nanoparticles [9,10], or fluorescent silicon nanoparticles quenched by nitroaromatic vapors enabled the development of optical sensors. The electrochemical activity of the nitro groups of TNT, for example, provided the basis of developing voltammetric sensors for this explosive [11,12], and recently, a composite of gold nanoparticles linked to electrodes enabled a sensitive electrochemical detection of TNT [13].
The redox activity of the nitro groups associated with many of the explosives was used to develop electrochemical sensors [14], and modified electrodes such as mesoporous SiO2-functionalized electrodes were employed to enhance the sensitivity of detection of nitroaromatic explosives [15]. Other electronic devices for the analysis of explosives included surface acoustic wave (SAW) systems. The coating of the piezoelectric devices with silicon polymers [16], carbowax [17] or cyclodextrin polymers [18] yielded functional coatings that enabled the electronic transduction of explosives adsorbed to these matrices; while the aggregation of functionalized gold nanoparticles in the presence of TNT was used to develop an optical sensor for the explosive [19].
The eliciting of antibodies that exhibit specific binding to nitroaromatics enabled the development of biosensors for explosives, using immunocomplexes as sensing units. This was exemplified with the development of TNT biosensors based on the displacement of the anti-TNT antibody from a surface-confined immunocomplex by TNT and the transduction of the dissociation of the antibody by surface plasmon resonance (SPR) spectroscopy [20-24] or quartz crystal microbalance (QCM) measurements [25]. Different optical [26,27] or voltammetric [28] sensors for RDX were also reported. These include the fluorescence detection of RDX with an acridinium dye [26], or by the application of NADH-functionalized quantum dots [29]. Also, a competitive fluorescence immunoassay for the detection of RDX was reported [30].
Although substantial progress was achieved in the sensing of explosives, the different analytical protocols suffer from insufficient sensitivity, lack of specificity, long analysis time intervals, and/or complex and expensive analytical protocols.
The unique electronic and optical properties of metallic and semiconductor nanoparticles, NPs, added new dimensions to the area of sensors. The aggregation of Au (gold) NPs as a result of sensing events and the formation of an interparticle coupled plasmon absorbance was used for the development of colorimetric sensors [31]. For example, color changes as a result of aggregation of Au nanoparticles were used to detect phosphatase activity [32], polynucleotides [33], or alkali (lithium) [34] ions. Also, the shifts in the plasmonic absorption bands associated with Au nanoclusters as a result of changes in the surface dielectric properties upon sensing were used to develop optical sensors for dopamine [35], adrenaline [36], cholesterol [37], DNA hybridization [38], and pH changes [39]. The layer-by-layer deposition of Au NPs on electrodes by the electrostatic cross-linking of the NPs by charged molecular receptors was used to construct electrochemical sensors for different neurotransmitters [40].
The imprinting of molecular recognition in organic or inorganic polymer matrices is known to permit generation of selective binding sites for the imprinted substrates [41]. Indeed, numerous optical [42] or electronic [43] sensors based on imprinted polymer matrices have been developed in the past two decades. For example, electrochemical sensors that consisted of imprinted organic [44] or inorganic [45] polymers were developed, and imprinted inorganic matrices associated with the gate surface of field-effect transistors were applied for the stereoselective or chiroselective analysis of the imprinted substrates [46]. Similarly, a quartz crystal microbalance [47] and surface plasmon resonance spectroscopy [48] were used as readout methods for the binding of substrates to the imprinted sites. The use of imprinted polymers as functional sensing matrices suffers, however, from several basic limitations. The density of imprinted sites is relatively low, and thus, for sensitive sensing thick polymer matrices are required. This leads, however, to slow binding of the analytes to the recognition sites (long analysis time intervals) and to an inefficient communication between the binding sites and the transducers. In fact, several studies suggested the use of imprinted monolayers [47], multilayers [48] and thin films to overcome these difficulties.